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CERN result provides answer to one of nature's most subtle secrets

21 Jun 1999

NA48 experiment

At a seminar at CERN* on 18 June Pascal Debu, spokesman of the Laboratory's NA48 experiment** , announced its preliminary result, after analysis of 10% of the expected data, on one of nature's best-kept secrets. Direct CP-violation, as it is called, is a subtle effect that betrays nature's preference for matter over antimatter, the reason why we are here. In 1993, an earlier CERN experiment, NA31, and the E731 experiment at Fermi National Accelerator Laboratory in the USA published the first precise results on direct CP-violation. The CERN result suggested that direct CP-violation was a real effect. The Fermilab result, while not excluding the effect, was also compatible with no direct CP-violation. More precise measurements were clearly needed, and ambitious new experiments at the two laboratories soon rose to the challenge. Both will now measure the effect with several times greater precision than their predecessors and the results of the KLOE experiment in Frascati (Italy) which will attempt a determination of the same effect with a completely different method are also eagerly awaited.

Fermilab's KTeV experiment reported a result in February compatible with the earlier CERN result. With Friday's announcement at CERN also confirming the original CERN result, direct CP-violation now seems beyond doubt***.

CP-violation is one of three conditions outlined in 1964 by Russian physicist Andrei Sakharov to account for the observed imbalance of matter and antimatter in the Universe. Without it, we simply would not be here. According to Sakharov, CP-violation is the result of a fundamental difference between matter and antimatter. It boils down to the concept of symmetry. Each of C and P are symmetries that are conserved in most particle interactions. C, for example, represents swapping the charges of all the particles in an interaction, in other words, swapping particles and antiparticles. If the interaction looks the same before and after, then C is said to be conserved, if not, C is violated. P is called parity and it corresponds to looking in a mirror which reverses all three spatial co-ordinates. Physicists once thought that each of these symmetries was conserved in particle interactions, but then in 1956 T.D. Lee and C.N. Yang demonstrated that P could be violated in weak interactions. The CP combination was then thought to be conserved, but this too proved not to be the case, and then came Sacharov's condition suggesting that under certain circumstances, CP ought not to be conserved.

CP-violation was first observed at the US Brookhaven laboratory by Christensen, Cronin, Fitch and Turlay in 1964 when their Nobel prize winning experiment showed that particles called long-lived neutral kaons occasionally decay into two pions, a CP-violating process. Then in 1973, Japanese physicists Kobayashi and Maskawa showed how to incorporate CP-violation into the theoretical framework of electromagnetic and weak forces. Their work pointed the way to the NA31 and E731 experiments and their successors NA48 and KTeV.

The NA48 experiment has been designed to detect tiny differences in the decay rates of neutral kaons and their antiparticles. The technique of the experiment involves simultaneously measuring the decays of two kinds of neutral kaons, long lived and short lived. By measuring both at once, run-to-run uncertainties are avoided and small detector inefficiencies cancel out in the final result. The experiment's 'pièce de résistance' is an energy measuring detector (a calorimeter) of 10 m3 using liquid krypton as its active medium. This allows NA48's physicists to pinpoint just those kaon decays that are interesting for the CP-violation analysis.

Footnote(s)

* CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and Unesco have observer status.

*** The quantity measured by these experiments is called '/. The 1993 NA31 result was (23±6.5) x 10-4, the E731 result was (7.4±5.9) x 10-4. The new results are (18.5±7.3) x 10-4 and (28±4.1) x 10-4 respectively from NA48 and KTeV, giving an overall average of (21.2±2.8) x 10-4.